Although modern oil and gas well production has progressed to a fine art, a variety of difficult problems may still be encountered during well completion, production, servicing and rework. Of necessity, these situations must be remedied from the well platform. Each well presents a unique challenge depending upon the well type, i.e., oil or gas, and the action to be taken. Typical problems requiring correction within a well are: crushed regions in the tubing, sand bridges or accumulation of paraffin, scale, rust or other debris. Maintenance procedures that must also be accomplished from the surface include the need to set or remove lock mandrels, collar stops or safety valves. Specific, commercially-available tools have been developed for each of these maintenance actions or problem solutions.
Although the actual tools may be very similar, at least three different prior art methods exist for powering the tools in the downhole environment. These methods for performing maintenance or for solving downhole problems are: wireline, pumpdown and plunger lift systems. As usual, each of these methods has both advantages and disadvantages.
Wireline systems use a variety of tools on the end of a wire acting through the flowline or wellbore. Various combinations of tools and accessories, e.g. swage, gauge cutter, broach, knuckle joint, stem, jars or accelerator, are assembled in a linear method creating a tool string which is used to perform the necessary action. Each tool string is custom-tailored to perform a required function. In wireline, a key part of the tool string is the stem, which is used to overcome stuffing box packing friction as the line passes from outside the well head into the well bore. With a set of mechanical jars below the stem, the combined weight of the jars and stem is used to jar up or down by pulling and then releasing the wireline. The combination of gravity and momentum, acting on the stem and jars, creates the force to perform the necessary function acting through the tool which is at or near the end of the tool string. In many cases, when the tool has successfully performed its function, additional jarring is used to shear a pin to release the tool from the work or newly-installed part. The tool string is then retrieved to the surface with the wireline. Wireline is most effective in near vertical wells, as the effects of gravity on the tool string diminish rapidly as the well bore departs from the vertical.
The most significant downhole problem that can occur with the use of a wireline is breaking of the wire, stranding a tool string in the flowline. This necessitates an additional maintenance action of retrieving the stranded tool string. This is usually accomplished by fishing with another tool string to acquire either the broken wire or the fishneck of the stranded tool string.
Pumpdown or through-flowline (TFL) service systems use hydraulic pressure and flow to provide the force required for tool movement and manipulation. In any TFL service system, there are five basic components: (1) a pump to provide power at the surface, (2) fluid to convert the pump power to work, (3) a circulation member to provide a complete circulation path, (4) a suitable conduit to carry the working fluid and (5) a tool string to perform the needed transport and service.
A TFL service system requires a fluid circulation path from a central service station into a well, through a communication port, and then returning to the point of origin. This path may be through the tubing/casing annulus, dual tubing strings or tubing side string in single or multiple zone completions. A hydraulic pump provides the hydraulic power and fluid flow to move the tool string through the circulation path to the desired depth in the well and to accomplish work downhole. A hydraulic manifold is controlled from the TFL operator's console at the surface. The manifold allows for the required fluid direction changes during a service action. Typical TFL service fluids are seawater, dead crude oil or diesel fuel. The TFL tool string consists of elastomeric piston units that convert the fluid flow into force to provide tool manipulation downhole.
TFL techniques are particularly useful for subsea completions, directional holes drilled from offshore platforms and/or deep, deviated holes where wireline work is sometimes impossible or, at best, extremely difficult. TFL provides additional power beyond that capable with wireline service equipment to cut through extreme paraffin deposits, jet-erode stubborn sand bridges and accomplish other downhole maintenance tasks effectively. The greatest drawback to TFL systems is the extended fluid circulation path which connects to the pump at the surface. Thus, both wireline and TFL systems require some physical connection to the wellhead.
Plunger lift systems have much narrower application than wireline or TFL systems. Plunger lift is used primarily to unload excess fluids from a gas well or to increase production on an oil well. In these systems a tubing stop is inserted (usually by wireline) in the flowline at a desired depth, and a bumper spring is installed above the stop. The plunger is allowed to free-fall to the bumper spring. The plunger expands to the inside diameter of the flowline and the gas in the well lifts the plunger. The plunger is designed to surface as a solid interface between the fluid column and the lifting gas. As the plunger rises to the surface, the plunger acts as a swab, removing liquids in the tubing string. When the plunger rises to the surface, the liquids and gases are diverted to separate flowlines. The plunger may be used repeatedly to remove successively more of the accumulated liquid in the well or may be retrieved from the wellhead. The system may be automated or manually controlled.
Unfortunately, each of the above-described systems fails to address automatic selection and deployment of tools appropriate to the job to be performed. Accordingly, what is needed in the art is an automated system capable of selecting and acquiring an oil/gas well tool, deploying the tool to the required location downhole, performing the required task and returning to the wellhead.